In recent years, biodegradable resins typified by polylactic acid have come into focus from the viewpoint of environmental preservation. The polylactic acid is one of the most heat-resistant and highly transparent biodegradable resins, and is mass-producible from a material derived from plants such as corn and sweet potato. Therefore, the polylactic acid is less expensive. Further, the polylactic acid is carbon-neutral, thereby contributing to reduction in environmental load. Thus, the polylactic acid is very useful.
However, products molded from the biodegradable resin alone are generally insufficient in strength. A known method for improving the strength is to disperse an inorganic filler such as talc or a phyllosilicate in the biodegradable resin. However, addition of the talc reduces the transparency of the product, and addition of a larger amount of the phyllosilicate deteriorates the appearance of the product due to agglomeration of the phyllosilicate. Therefore, the addition of the talc or the phyllosilicate is disadvantageous for production of containers, sheets and films.
To solve this problem, a method for increasing the dispersibility is disclosed in JP-A-2004-027136, in which the polylactic acid and a phyllosilicate preliminarily swelled with water or an aqueous solvent are kneaded at a temperature not higher than the melting point of the polylactic acid. In JP-A-2004-204143, a method for improving the transparency is disclosed, in which a biodegradable resin containing hectorite or saponite is kneaded under specific conditions. In JP-A-2004-323758, a method for improving the dispersibility is disclosed, which employs a phyllosilicate having an interlayer distance increased by intercalation of a lactide or a lower molecular weight polylactic acid having a number average molecular weight not greater than 50,000.